1. Field of the Invention
The present invention relates to a vector network analyzer (VNA). More particularly, the present invention relates to measurement of s-parameters, third order intercept, harmonics, group delay, and noise figure using the VNA. The present invention further relates to measurement of three port devices, such as a mixer using the VNA, and calibration of the VNA.
2. Description of the Related Art
FIG. 1 shows a block diagram of components included in a conventional vector network analyzer (VNA). The VNA is configured to make vector measurements, including both magnitude and phase, of a device under test (DUT) connected across its two test ports A and B. The VNA includes a signal source which generates RF/microwave signals and can sweep over a range of frequencies. A switch selectively connects the signal source to one of two reflectometers A or B. The reflectometers A and B include couplers which couple a reference RF signal as provided from the signal source to one of test ports A and B, and a test RF signal received at one of the test ports A and B. The couplers are connected to downconverters in the reflectometers which downconvert the test and reference RF signals to intermediate frequency (IF) signals. Further components in the reflectometers then process the IF signals to provide full two port error corrected scattering parameters (S-Parameters), and other measurements such as group delay.
A conventional VNA may also include components to measure noise figure as illustrated in FIG. 2. The noise figure is a way of specifying the noise parameters of a component, and is defined as the ratio of the signal-to-noise ratio available from the device output to the signal-to-noise ratio delivered to the input of the device, at a standard reference temperature of 290K. Noise figure measurements are made using a noise source and receiver. The noise source typically includes a noise diode connected to a power supply. The noise source provides noise over a wide bandwidth to one port of a DUT, while the other port is connected to a receiver. The receiver downconverts the noise source signal from the DUT to an intermediate frequency (IF) range, and the IF signal is processed to provide an indication of power level enabling the noise figure to be determined over the frequency range.
In FIG. 2, the noise source in the VNA is connectable by a switch A through a reflectometer A to the measurement port A of the VNA. The switch A normally connects the signal source of the VNA through the reflectometer A to the test port A for standard VNA measurements and is switched to the noise source when noise figure measurements are desired. Similarly, a receiver is connectable by a switch B to the test port B of the VNA. The switch B then normally connects to the signal source for standard VNA measurements and is switched to the receiver when noise figure measurements are desired.
The receiver in FIG. 2 is separated from the reflectometers A and B, but its components are the downconverters and IF processor typically included in the reflectometers. Rather than use two separate receivers, switching is included in the receiver so that signals can be downconverted and processed from the reflectometers as well as the noise source stimulated DUT when the switches A and B are so configured.
Noise figure measurements have been performed for years with a wide variety of instruments, but the instruments have lacked flexibility. Usually the noise source and receiver must be attached directly to the DUT for all measurements, or the noise source is provided internally in a VNA, as shown in FIG. 2, for all measurements. The user, thus, has little choice with respect to test setup, noise source selection, or noise source traceability. With the receiver measuring power, and not operating in ratioed mode, the user cannot usually apply vector corrections using the VNA to compensate for DUT or system mismatches when the noise figure measurements are made.
Devices for measuring noise figure also do not allow a selection of frequency bandwidths for measurements which may limit the types of DUTs that can be measured using one test device. The measurement bandwidth being fixed for a given test system may also lead to either inordinately long test times or to inaccurate results.
A VNA may be used with an external automatic calibration as illustrated in FIG. 3. The external calibration device can include calibration components include elements such as a short, open, match and a thru connection which are selectively connectable by switches to test terminals which can be connected to test ports A and B of the VNA. The automatic calibration device can also include verification lines which are connectable by switches between its terminals to verify calibration once calibration is complete. The internal components are initially characterized using actual short, open, match and thru lines which are directly connected to the ports of the VNA. During calibration using the external calibration components, calculations are performed to account for imperfections in the calibration standards of the automatic calibration device based on measurements which are made and stored when measuring the actual calibration standards.
The automatic calibration device can be provided with a separate controller, as further shown in FIG. 3 which connects to the VNA and automatic calibration device to automatically control the calibration process. The VNA typically has a keypad which a user uses in conjunction with the external controller to type in commands to set up and run each calibration step after connecting a different standard. The controller is programmed to function as the user, and sends information to the VNA""s processor to set up and run each calibration step after each calibration component is connected by internal switches in the automatic calibration device to the terminals of the VNA. The automatic calibration process is started by a user depressing a start button on the controller keypad. An automatic calibration device with components as described above, including a controller is described in U.S. Pat. No. 5,587,934 entitled xe2x80x9cAutomatic VNA Calibration Apparatusxe2x80x9d, to Oldfield, et al. which is incorporated herein by reference.
Since a conventional VNA includes only two ports and a single signal source, measurement of components requiring two input signals, such as a mixer, and measurements of parameters where two input signals are applied, such as second and third order intercepts, cannot be performed with a conventional VNA alone.
A typical set up for measuring the frequency translation parameters of a mixer is shown in FIG. 4. A first of two input signals f1 is provided to the mixer from a VNA. Since the VNA only has one signal source, apart from its local oscillator, a second signal f2 is provided from an external signal generator. The output of the mixer is then measured using a spectrum analyzer. The output of the mixer provides a frequency translation including a sum and difference of its input signals f1xc2x1f2 along with higher harmonics of those signals at its output. The spectrum analyzer is used to measure the mixer output since the reflectometers of a VNA are typically configured to measure scattering parameters rather than to provide the function of a spectrum analyzer. Using the VNA to provide the input to one port of the mixer will allow use of the VNA to characterize one port of the mixer without reconfiguration of the mixer test setup.
A typical test set up for measurement of second or third order intercept is shown in FIG. 5. Second and third order intercept measurements are a way of characterizing distortion. Because of the increasing need for wide dynamic range at high frequencies, most wideband amplifiers, as well as other microwave and millimeter wave components, now have distortion specification. When two tones, or signals, are applied to an amplifier that is non-linear, the nonlinearity causes them to modulate one another, producing intermodulation distortion (IMD). The higher the second and third order intercept power levels, the higher the input level at which IMD becomes significant, and the lower the IMD will be at a given signal level. Second and third order intercept points are, thus, now typically specified for amplifiers.
FIG. 6 shows a plot illustrating how second and third order intercept points are derived. For measurements, two spectrally pure signals are applied to the amplifier. The output signal power in a single signal (in dBm) and the relative amplitudes of the second-order and third-order products referenced to the single signal and then plotted as shown in FIG. 6. Extrapolation of the measurements are also plotted as shown in dashed lines as a function of input signal power. Beyond a certain level, the output fundamental signal begins to limit, or compress. If the second and third order IMD lines are extended, they will intersect the extrapolated extension of the fundamental output line. The projected intersections are called the second and third-order intercept points.
As shown in FIG. 5, the setup for measurement of second or third order intercept, thus, includes two signal sources for providing two tone signals to a DUT and a spectrum analyzer for measuring the responsive outputs from the DUT. One of the two signal sources in FIG. 4 can be the VNA, as shown, enabling the VNA to be used for one port characterization without disturbing the test setup. Since the VNA only has one signal source, the second signal source is provided by a separate signal generator. The signals from the two signal sources are combined and provided to the single input of the DUT using a combiner. The combiner can be a coupler, power divider, or one of a variety of other types of devices. The output of the DUT is then connected to the input of the spectrum analyzer for measurement of the DUT output power. A combination of the DUT output power and the measured power level from one of the signal sources can be used to determine second and third order intercepts.
VNAs are typically configured to measure group delay using the formula dxcfx86/dxcfx89, where xcfx86 is the phase of an S21 transmission measurement. However, the measurement technique fails for frequency translating devices, such as mixers, since accurate and stable phase information is not available in these generally unratioed measurements, and VNAs are programmed make measurements in a ratioed manner. Presently, additional components such as reference path mixers must be used with a VNA to make frequency translated group delay measurements.
To reduce the amount of test equipment needed for any given microwave or millimeter wave test measurement, it would be desirable to provide a VNA that can measure second and third order intercept, group delay for frequency translating devices, or that can provide full error corrected characterization of any three port device with limited additional test components. With two signals being provided to a DUT, it is further desirable that common mode noise generated by the two signals not affect measurement accuracy, and that the signal sources have a fast relative settling time to provide a fast measurement speed.
With the dynamic range of DUTs which are typically tested increasing, it is further desirable to provide a VNA which can provide more accurate measurements over the dynamic range. In particular, with higher harmonic output measurements required from DUTs, it is desirable that measurements of harmonics of the DUTs be free of source harmonics.
In accordance with the present invention, a VNA is provided with a complete integration of various measurement capabilities into one optimized system, including full vector error corrected 3-port S-parameters measurements, second and third and higher order intercept measurement capability, frequency translation mixer measurement capability including frequency translating group delay measurement capability. The VNA further includes components to enable noise figure to be determined and to enable automatic calibration. The VNA further includes an enhanced measurement procedure to enable the harmonic response of a DUT to be measured over a wide range without limitations from the stimulus source harmonic level. The VNA further includes connections for a multiple source/LO module which provides multiple signals with limited common mode noise, and provides fast relative settling times for the sources, enabling faster system measurement speed.
The VNA includes three test ports with a first signal source connectable to two of the test ports through reflectometers, and a second signal connectable to the third test port through a reflectometer to enable full error corrected 3-port S-parameters measurements to be performed on a three port device.
The VNA includes software to enable it to function as a spectrum analyzer in a non-ratioed mode so that with use of the VNA with a coupler and the two signal sources, second and third order intercept measurements can be made. The two signals sources may also be connected through test ports of the VNA to two inputs of a mixer, and act in a non-ratioed mode to enable mixer frequency translation measurements to be made without requiring external test components.
With a signal source, an internal modulation synthesizer special IF circuits and appropriate software, frequency translation group delay measurements can be made using the VNA. To measure group delay, the the main carrier of the first signal source is modulated using the modulation synthesizer, and the phase change of the modulated signal is observed. Since the modulating frequency in most DUTs does not change, the measurement can be ratioed against the original modulating signal to extract phase data.
The VNA further includes components/software to enable it to provide the function as the controller for the automatic calibration device as shown in FIG. 3. The VNA keypad is then used entirely to control the automatic calibration process once the automatic calibration device is attached, and control signals are provided from a control connection between the VNA and automatic calibration device to control its switches.
The VNA further includes an additional external connection port for a noise source to allow user selection of different noise sources connected directly to a DUT, or alternatively connected through a VNA to make noise figure measurements more flexible. The VNA also includes a receiver with a downconverter IF output selectively provided through one of two channels, a wideband channel and a narrowband path. The wide IF path will provide the fastest measurements, but may be inappropriate for some narrowband DUTs. The narrowband path is available for measuring the noise figure of very narrowband devices such as devices operating over less than a 1 MHZ bandwidth.
The VNA further includes software for determining the harmonic response of a DUT which eliminates the effects of stimulus source harmonics. For the software, the vector sum of the DUT responses GHx to the source tuned to the fundamental with its associated harmonics is first measured. The harmonics from the DUT GNx with the source tuned to the fundamental are then obtained. Vector subtraction is then used to determine the output harmonic of the DUT, Hx, as follows Hx=GHxxe2x88x92GNx. The output harmonic Hx will be free from source harmonics.
The VNA can use a dual mode multiple source/local oscillator module to provide its two sources and local oscillator. The module includes both coarse and fine signal sources. The coarse signal sources can be connected together in a common offset mode, or can operate separately in an independent offset mode. Operation in the common offset mode provides greater dynamic tracking with a reduction of receiver If phase noise and improved IF settling time.